Trailers and trucks are equipped with boxes for carrying products and bulk material. The box mounted on the frames of the trailers and trucks have upright side walls and upright front and rear end walls. The end wall may have doors to provide access into the box. The side walls are constructed of material as wood or metal and are anchored to the outside edges of the floor. Boxes made of wood will warp when exposed to moisture and will splinter and break when subjected to impact forces. Metal boxes are relatively heavy, thereby decreasing the load-carrying capacity of the truck. An example of a trailer box is shown in U.S. Pat. No. 3,733,089.